There are buses from Garda (the town) that run all the way to Torbole but the buses from the bottom of the lake - Peschiera del Garda. Sirmione - stop at Malcesine necessitating a change.
Bus map here: https://www.atv.verona.it/en/LineeLago
There are also ferries but a ferry the length of the lake is probably 3+ hours long and again would probably mean a change at Malcesine.
Ferry planner here: https://www.navigazionelaghi.it/en/tickets-and-timetables-lake-garda/
I think Roberto's solution is the best - especially because all these solution only get you to Torbole not Nago-Torbole and even though it's very close it means one more connection. North of Malcesine transportation options are reduced.
Hope this turns out to be simple for you, have a great trip,
=Tod